Some of the operations of a floating vessel used for drilling operations (e.g. a semi-submersible drilling rig or a drill ship) are impeded by sea swell. Sea waves impart an up and down motion to the vessel (known as ‘heave’), the period of which can range from a few seconds to 25 s or so, and can be of a few centimeters to 15 m or more in amplitude. This up and down motion is imparted to a load attached to the vessel. In many circumstances the motion of the load is highly undesirable and even dangerous to equipment and personnel. For example when attempting to drill a wellbore in the sea floor, the motion can cause a corresponding motion of the drill string. The up and down movement of the drill bit is highly undesirable and severely restricts the operating window of the rig. For example, it is estimated that in the North Sea as much as 20% of rig operating time is lost ‘waiting on weather’ i.e. waiting for better weather when the sea is calmer.
Active heave compensation is concerned with reducing the effect of this up and down motion on a load attached to the vessel via a connecting device (e.g. a travelling block, top drive, or the like). So-called ‘passive’ active heave compensation methods are known which rely on the load being fixed at some other point (e.g. to the sea floor). Sea swell causes the vessel to move relative to the load and a passive compensator uses compressed air to provide a low frequency damping effect between the load and the vessel. There are several disadvantages with passive heave compensation methods and apparatus, including that the weight (typically 100-150 tons) of the passive compensator is typically suspended tens of meters above the rig floor, which affects the center of gravity of the vessel, and that the use of passive compensation is limited to loads that are attached to some other point.
So-called active heave compensation methods have been deployed in the field in recent years. An active heave compensation method involves measuring the movement of the vessel using a measuring device (for example a Motion Reference Unit or MRU) and using a signal representing the motion of the vessel to control a drive for moving the connecting device relative to the vessel. In principle, if the connecting device is moved in a manner equal but opposite to the motion of the vessel the heave can be substantially cancelled. A major advantage of active heave compensation is that it does not rely on movement of the load itself relative to the vessel before compensation can be applied.
It will be appreciated that while some operations of floating vessels are impeded by heave, other operations are impeded to a lesser degree or not at all. For example, when raising or lowering loads to/from the seabed, the hoisting operation is only impeded by heave when the load approaches the sea floor, i.e. when the height of the load above the sea floor is of the order of, or less than, the maximum heave.
Consequently it is often desirable to operate a hoisting system in a first mode without active heave compensation and in a second mode with active heave compensation. For the purpose of the present description, the first mode will also be referred to as hoisting mode, while the second mode will also be referred to active heave compensation mode.
In both modes, it is generally desirable to provide emergency brakes for stopping the upward and/or downward motion of the connecting device (and thus of the load attached thereto) during failure situations, e.g. in case of malfunctioning of the drive that controls the motion of the connecting device.
Such emergency brakes are provided so as to avoid losing control over the load and/or vessel in situations of failure. For example, the emergency brakes prevent heavy loads, such as blowout preventers (BOPs), from descending to the sea floor in an uncontrolled fashion. Such emergency brakes may e.g. be disc brakes or another suitable form of brakes.
It is generally desirable to provide a floating vessel that provides efficient yet safe operation under most or even all operational conditions.